feat: add Agrarmonitor correspondent sync

- Extract getOrCreateCorrespondent helper to deduplicate logic
- Add syncCorrespondentIds to match Paperless correspondents to
  Agrarmonitor IDs via Lieferantennummer and persist in CorrespondentSetting
- New POST /api/agrarmonitor/sync-correspondents endpoint
- "Agrarmonitor-Abgleich" button in Correspondents settings tab

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

paperless-backend/src/agrarmonitor/agrarmonitor.controller.ts

@@ -49,4 +49,11 @@ export class AgrarmonitorController {
   async processUploads() {
     return this.pollingService.processVerarbeiteteDocuments();
   }
+
+  @Post('sync-correspondents')
+  @HttpCode(200)
+  @RequirePermissions(Permission.MANAGE_SETTINGS)
+  async syncCorrespondents() {
+    return this.pollingService.syncCorrespondentIds();
+  }
 }